Process for metal plating a stainless steel

ABSTRACT

A process for metal plating a stainless steel comprising the first step of treating the stainless steel by a cathode electrolysis in an aqueous solution containing free HCl in an amount at least 30 g/l and at least one species of Ni and Co in an amount at least 0.1 g/l; the second step of metal plating the same by Ni or a Ni alloy in a weakly acidic Ni plating bath; and the third step of subsequently plating the same by a noble metal, Cu or an alloy thereof.

DETAILED DESCRIPTION OF THE INVENTION

1. Field of the Invention

This invention relates to a process for metal plating a stainless steel(hereinafter referred to as SUS), which attains a high quality suitablefor use in the precision machinery industries and electronicsindustries. Particularly, the products are most suitable for the noblemetal plating use and can be utilized as contact points and connectionpart materials and the like.

2. Prior Art

SUS's are used for various purposes because they are generally inaustenite systems, ferrite systems, deposition-cured systems, every oneof which is excellent in physical characteristics including physicalstrengths and also in corrosion resistance due to the strong passivefilm formed on the surface. However, the formation of the passive filmnot only inhibit the junction characteristics to solders or brazingmaterials, but also causes a difficulty in electric connectings and,therefore, foreign metals especially noble metals, such as au, Ag, Pt,Pd, Ir and the like, are plated on them when they are used in precisionmachines and electronic instruments. These noble metals, which areexcellent in corrosion resistance, are suitable for the soldering andelectric connecting and are widely used in electric contact points,semiconductors and the like.

The formation of the passive films provides a considerable hinderance inthe metal plating operation, and it is necessary to remove the passivefilms to activate the surface. As processes suitable for this purpose,there have been known a process in which a material to be metal-platedis immersed in a solution of HCl, H₂ SO₄, or the like; a process for afurther strong activation in which an electrolytic treatment is carriedout in the solution using the material as a cathode and a process inwhich a Ni strike plating is applied. Usually, the plating is carriedout after the Ni strike plating is applied.

As the former process, it is immersed in a solution containing 1.75 g/lof HCl and 10 g/l of CH₃ COOH at 30° C. for 5 to 10 minutes or iselectrolyzed in a bath containing HCl in an amount of 100 g/l at 1A/dm²for 5 to 10 minutes to dissolve or reduce the passive film.

The Ni strike plating means, for example, a SUS material to be plated istreated as the cathode in an aqueous solution containing 240 g/l ofNiCl₂ and 80 to 120 g/l of HCl at a current density of 20 A/dm² for 2 to4 minutes to reduce the passive film by the electrolysis andsimultaneously plating Ni on the SUS surface in a thickness of 0.4 to 1μto protect the surface.

Problems to Be Solved by the Invention

There are problems when the SUS's which are plated by noble metals afterthey are strike plated by Ni, are used in precision machines orelectronic instruments as follows. There are many cases in that it isdifficult to plate them with metals after they are pressure molded ascontacting elements for use as, for example, spring contacts such as ofswitches and connectors, because they are small in size and complex inform and, in addition, they are contained in structural bodies.Furthermore, it is also desired in view of the processability to processthem by molding, after the SUS materials are previously plated. However,fine cracks tend to occur in the mold processing of the contactingelements, in which much processing, such as bending, extruding, drawing,and so on are involved.

These cracks are the cause of reductions in physical strengths and ofchanges in the electric contact resistance with the passage of time.This is because the Ni strike plating, accompanying the generation of agreat amount of H₂ results in an inclusion of excess H₂ in the plated Nilayer which hardens the Ni layer and further causes generation ofstresses with the result that cracks are generated in the plated Nilayer in the mold processing. On the other hand, the process asmentioned above, in which they are plated after being activated by atreatment involving a cathode electrolysis in an aqueous solution of HClor H₂ SO₄, has been performed for many years. However, products of thisprocess are inferior to those of the Ni strike plating process inreliability, because the surface is oxidized during the moving from theactivation to the metal plating, to again passivate. A process disclosedin Japanese unexamined patent publication No. 87296/1983 uses a specialplating bath for this reason, in which plating bath special organiccompounds, for example, a pyrrolidone derivative, acetylene glycoholhomologues, a nonion surfactant and the like, are combined in an acidicbath. However, even in this process, not only is the generation ofcracks inevitable, but also there is found the embrittlement in the SUSsubstrate itself. This is a class of hydrogen embrittlement caused bythe nascent hydrogen generated in a considerable amount on the SUSsurface and partly absorbed into the interior. This is remarkable inSUS's of the deposition cured systems of martensites. Further seriousdrawbacks are that adhesion characteristics degrade with the passage oftime to promote, for example, the delamination of layers plated by Au onpressure molded articles from peripheral parts during long periods ofuse.

Means for Solving Problems

This invention is, as a result of various investigations to solve thedrawbacks of the above mentioned conventional processes and to develop aprocess for metal plating a SUS which makes it possible to provide highquality platings of Cu and noble metals suitable for use in precisioninstruments and electronic instruments. This process comprises a noblemetal plating of SUS's of, applying a cathodic electrolysis treatment toa SUS material to be plated in an aqueous solution containing at least0.1 g/l of Ni or Co and at least 30 g/l of free hydrochloric acid,subsequently electrically plating Ni or a Ni alloy in a weakly acidicplating bath, and then conducting a plating with Cu or a noble metal.

That is to say, this invention is to apply the following processings toa SUS to be plated, before the plating. In addition, a treatment fordegreasing or removing scales can be carried out before thesetreatments, when necessary.

The first processings is that a SUS material for use in plating istreated by a cathode electrolysis in an aqueous solution containing notless than 0.1 g/l of Ni or Co and not less than 30 g/l of freehydrochloric acid using a Fe 1˜50% Ni or Co, such as a FeNi, FeCo,FeNiCo or SUS alloy as an anode, in which an electric current density of1 to 100 A/dm² on the cathode and a processing time of 1 to 180 secondsare controlled within the range in relation to the bath composition.

The second processing is that the SUS material for use in the platingwhich has been processed in the first step is washed by water and isplated with Ni or a Ni alloy, for example, a Ni--Co (the Co contentranging from 5 to 20%), Ni--Zn, Ni--Fe, Ni--P (the P content rangingfrom 1 to 5%) or the like to a thickness of 0.05 to 0.5μ using a weaklyacidic plating bath. As the weakly acidic plating bath, one such as aNiSO₄ bath, sulfamic acid bath, borofluoride bath or the like, which hasa pH value of 2˜4 is used.

The plating successively carried out after the above processing is donewith Cu, the above mentioned noble metals or an alloy thereof, forexample, PdNi, PdCo, AuCo, AuNi, AuSb, AuAgCu, PdAg, AgCu, AgZn, AgSb,CuNi, CuSn, CuZn, or the like in the conventional way.

Action

The SUS material to be plated is activated on the surface in the firststep processing and at the same time, a micro amount of metal containingNi or Co deposits which protects the SUS material on the surface, so itdoes not again convert to the passive state. However, if the amount offree hydrochloric acid is less than 30 g/l, not only the activation isinsufficiently attained, but also the deposition of embrittled Nioccurs, which is disadvantageous. When the amount of Ni is less than 0.1g/l, the suppression of the repassivation and the above mentionedhydrogen embrittlement is insufficient, and a stable adhesion of Niplating layer can not be obtained. These amounts are desirably not lessthan 100 g/l of free hydrochloric acid and not less than 5 g/l of Ni. Itis one of merits of SUS or Fe--1˜50% Ni alloy, used as the anode in thisprocessing, that the generation of a furiously poisonous Cl₂ gas isprevented, which Cl₂ gas is generated when an insoluble anode, such ascarbon or Pt is used, but not when the Fe--1˜50% Ni or Co alloy is usedbecause it is soluble. Moreover, not only a supply of Ni or Co componentis attained, but also the dissolved Fe or Cr simultaneously exertsunexpected effects. Namely, as contrasted with a single bath compositionof HCl and NiCl₂ which gives a deposition of hard and thick Ni layersimilar to that in the case of the conventional Ni strike plating, aNi-rich layer of 300˜300 Å thickness having Ni content of 10˜60% givenby the augean spectroscopic analysis is formed in a solution in which aSUS or Ni--Fe alloy is dissolved in a metallic concentration, forexample, 20 g/l. The reason for this is not clear, but it is presumedthat an excessive deposition of Ni or Co may be suppressed by thedeposition of Ni-Fe alloy. Furthermore, the above mentioned Ni-richlayer effectively prevents the repassivation and makes it possible toprovide a Ni or Ni alloy plaing which is excellent in adhesiveness inthe second processing. A thin metal layer is deposited in the process ofthis invention, which results in little adsorption of hydrogen and thehydrogen embrittlement of SUS of a martensite system or depositioneffect type can be suppressed. Reasons why an anode of Fe--1˜50% Ni orCo is especically recommended in this application have been partlydescribed above. The lower limit of Ni or Co is set forth as not lessthan 1%, because the cathodic deposition efficiency may have a lowervalue not more than 10% in many cases, as compared with the currentefficiency of anodic dissolution which may be in a value near 100%. Forthis reason, Ni sufficiently in excess of the 50% Ni or Co results in awaste of expensive Ni or Co.

The second processing is to carry out the Ni or Ni alloy plating sothere is no generation of cracks in the pressure molding or the like,and to suppress the delamination of Cu or a noble metal plated on the Nior Ni alloy layer during long time periods of use. The Ni or Ni alloyplating layer, which has a hardness (Hv) of around 200 to 300 in anycase, is soft and abundant in flexibility, as compared with a hardness(Hv) not less than 400 of the conventional Ni strike plating, which hasa large amount of occluded hydrogen.

Delamination occurs in the above mentioned conventional articles platedby Cu or a noble metal when used for long periods. This is considered asa class of electric corrosion effects. In contrast to this, it isconsidered that the intermediate layer of this invention comprising Nior a Ni alloy, which is positioned in the middle of a great electricpotential difference between the active SUS and the layer of the noblemetal or Cu, greatly suppresses the electric corrosion in the interface.The Ni or Ni alloy layer is practically set forth as not less than0.05μ, and desirably ranging from 0.07 to 0.25μ, because the layerexceeding 0.5μ accelerates the generation of cracks.

The above mentioned Ni or Ni alloy plating layer is deposited from abath having a pH ranging from 2 to 4, especially and desirably from 2.5to 3.5. A pH exceeding the range results in the hardening andembrittlement due to the absorption of hydrogen, and the occlusion of ahydroxide of Ni and the like. The effects of this invention can beespecially maximized when a bath containing Ni sulfamate as the maincomponent is used, namely, a bath containing 200 to 600 g/l of Nisulfamate and 10 to 50 g/l of H₃ BO₃, and having a pH of 2 to 4.

As explained above, this invention has solved the disadvantages in theconventional noble metal or Cu plating on a SUS, in that the SUS isactivated on the surface and temporarily protected at the same time byconducting a two step pretreatment in advance of the Cu or noble metalplating, so as to make it durable to a complexed mold preocessing andmaintain high quality when used for a long period, of time bysubsequently plating soft Ni or a Ni alloy followed by the plating. Theplating can be applied, so as to give multilayers when necessary. Forexample, it can contribute to the improvement of the soldering and theadhesiveness of an Ag plating layer at a high temperature if Cu isplated for the first layer and Ag for the second layer. For anotherexample, a first layer plated with Pd and a second thin layer platedwith Au exert the equivalence of a thick Au plating layer incharacteristics as contact points and have economic merits.

EXAMPLES

<1> SUS 310 of 0.12 mm thickness was used , and after this was degreasedby acetone, the various processings shown in Table 1 were appliedthereto. Then, noble metal platings were carried out to give layers of1.0μ in thickness. The conditions of the plating baths indicated in thisTable are shown in Tables 2 and 3. As to these samples, tests for theprocessability and the long term adhesiveness were carried out. Resultsare shown in Table 4.

The test for processability was carried out by extruding the work usingpressure molds to prepare specimens of 8 mm in diameter and 0.3 mm and0.6 mm in height. A part of them was subjected to the brine sprayingtest for 4 hours according to JIS (Japanese Industrial Standard) Z 2371and then, the presence or absence of rust occurring on the processedpart was visually observed. Some other parts were pressed on a Au plateby the pressure of 50 gG, where a DC is charged in a rate of 100 mA, tomeasure the electric contact resistance after they were kept in amoisture chamber at a temperature of 80° C. and a humidity of 95% for1000 hours.

The adhesiveness was measured as follows:

Lines reaching the SUS substrates were cut by a cutter knife in acheckers figure having intervals of 1 mm on the specimens, which werethen kept in a pressure cooker chamber at a temperature of 120° C. andhumidity of 90% for 2000 hours. The delamination test was carried outaccording to JIS D 0202 method using an adhesion tape and thedelamination situations of the plated parts were visually observed.

As it is obvious from Tables 1 and 2, cracks were generated by thepressure-processing to result in the significant generation of the rust,due to the brine and high contact resistance in Comparative Test No. 16,in which the strike plating by Ni was used, because the products wereinferior in the processability. In contrast, it is seen from Examples 1to 9 that the products of this invention were excellent inprocessability, prevented rust generation by the brine, showed lowelectric contact resistance, and no delamination of the noble metallayers was observed during long time periods of use.

                                      TABLE 1                                     __________________________________________________________________________              First Processing               Second Processing                              HCl                                                                              NiCl.sub.2                                                                        Ni         Current Density                                                                        Time                                                                              Plating                                                                            Plating                                                                              Thickness                                                                           Metal                     No.                                                                              (g/l)                                                                            (g/l)                                                                             (g/l)                                                                             Anode  (A/dm.sup.2)                                                                           (Min.)                                                                            Bath Metal  (μ)                                                                              Plating            __________________________________________________________________________    Present                                                                              1  100                                                                              --  11  SUS 301                                                                              5        0.5 Bath A                                                                             Ni     0.25  Au                 Invention                                                                     Present                                                                              2  100                                                                              --  19  SUS 301                                                                              5        0.5 Bath A                                                                             Ni     0.25  Au                 Invention                                                                     Present                                                                              3  100                                                                              --  6   SUS 301                                                                              5        0.5 Bath A                                                                             Ni     0.1   Au                 Invention                                                                     Present                                                                              4   45                                                                              --  5   SUS 631                                                                              12       0.5 Bath B                                                                             Ni--10% Co                                                                           0.08  Ag                 Invention                                                                     Present                                                                              5   35                                                                              --  0.2 Fe--5% Ni                                                                            2.5      2.0 Bath C                                                                             Ni      0.075                                                                              Ag                 Invention                                                                     Present                                                                              6   35                                                                              --  0.8 Fe--5% Ni                                                                            2.5      2.0 Bath C                                                                             Ni     0.15  Ag                 Invention                                                                     Present                                                                              7  100                                                                              --  Co 1.5                                                                            SUS 301                                                                              5        0.5 Bath C                                                                             Ni     0.15  Pd                 Invention                                                                     Present                                                                              8  100                                                                              --  Co 7.5                                                                            Fe--5% Co                                                                            1.5      0.5 Bath A                                                                             Ni     0.5   Au                 Invention                                                                     Present                                                                              9  100                                                                              --  --  Fe--15% Co                                                                           1.5      0.5 Bath A                                                                             Ni     0.5   Ag                 Invention                                                                     Comparative                                                                          10 120                                                                              --  --  Pt     5        1.0 Bath C                                                                             Ni     0.15  Au                 Test                                                                          Comparative                                                                          11 100                                                                              --  0.07                                                                              Pt     5        1.0 --   --     --    Au                 Test                                                                          Comparative                                                                          12 120                                                                              --  16  Pt     5        1.0 Bath C                                                                             Ni     0.15  Au                 Test                                                                          Comparative                                                                          13  15                                                                              --  11  SUS 304                                                                              5        1.0 Bath C                                                                             Ni     0.15  Au                 Test                                                                          Comparative                                                                          14 100                                                                              --  11  SUS 301                                                                              5        0.5 Bath A                                                                             Ni     0.01  Au                 Test                                                                          Comparative                                                                          15 100                                                                              --  11  SUS 301                                                                              5        0.5 Bath A                                                                             Ni     0.75  Au                 Test                                                                          Comparative                                                                          16 120                                                                              240 --  Ni     10       0.05                                                                              --   --     --    Au                 Test                                                                          __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                        Bath A (Sulfamic Acid Bath)                                                   Ni(SO.sub.4 NH.sub.2).sub.2                                                                       500 g/l                                                   NiCl.sub.2          25 g/l                                                    H.sub.3 BO.sub.3    30 g/l                                                    pH                  3.0                                                       Temperature of Bath 55° C.                                             Current Density     5 A/dm.sup.2                                              Bath B (Ni--10% Co Bath)                                                      NiSO.sub.4          250 g/l                                                   NiCl.sub.2          30 g/l                                                    CoSO.sub.4          20 g/l                                                    H.sub.3 BO.sub.3    30 g/l                                                    pH                  2.9                                                       Temperature of Bath 55° C.                                             Current Density     3 A/dm.sup.2                                              Bath C (NiSO.sub.4 Bath)                                                      NiSO.sub.4          250 g/l                                                   NiCl.sub.2          30 g/l                                                    H.sub.3 BO.sub.3    30 g/l                                                    pH                  3.2                                                       Temperature of Bath 50° C.                                             Current Density     2.5 A/dm.sup.2                                            ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Bath for Plating Au                                                           Bath N-40 (Manufactured by Japan Engelhalt Co.)                               Temperature of Bath                                                                             55° C.                                               Current Density   0.25 A/dm.sup.2                                             Bath for Plating Au                                                           AgCN              60 g/l                                                      KCN               60 g/l                                                      K.sub.2 CO.sub.3  25 g/l                                                      Temperature of Bath                                                                             30° C.                                               Current Density     2 A/dm.sup.2                                              Pd-20Ni Plating Bath                                                          Bath PNP-80 (Manufactured by Nisshin Chemical Co., Ltd.)                      Temperature of Bath                                                                             25° C.                                               Current Density   0.5 A/dm.sup.2                                          

                                      TABLE 4                                     __________________________________________________________________________              Generation of Rust                                                                            Electric conduct                                              by the Brine    Resistance (mΩ)                                                                     Adhesiveness                                      Height of       Height of   Before the                                                                             After the                             No.                                                                              0.3 mm  0.6 mm  0.3 mm                                                                              0.6 mm                                                                              Test     Test                           __________________________________________________________________________    Present                                                                              1  No generation                                                                         No generation                                                                         5.9   6.6   No delamination                                                                        No delamination                Invention                                                                     Present                                                                              2  No generation                                                                         No generation                                                                         6.1   7.1   No delamination                                                                        No delamination                Invention                                                                     Present                                                                              3  No generation                                                                         No generation                                                                         5.4   5.3   No delamination                                                                        No delamination                Invention                                                                     Present                                                                              4  No generation                                                                         No generation                                                                         12.0  11.0  No delamination                                                                        No delamination                Invention                                                                     Present                                                                              5  No generation                                                                         No generation                                                                         13.0  11.0  No delamination                                                                        No delamination                Invention                                                                     Present                                                                              6  No generation                                                                         No generation                                                                         11.0  14.0  No delamination                                                                        No delamination                Invention                                                                     Present                                                                              7  No generation                                                                         No generation                                                                         6.9   9.1   No delamination                                                                        No delamination                Invention                                                                     Present                                                                              8  No generation                                                                         No generation                                                                         5.8   6.9   No delamination                                                                        No delamination                Invention                                                                     Present                                                                              9  No generation                                                                         No generation                                                                         13.5  13.0  No delamination                                                                        No delamination                Invention                                                                     Comparative                                                                          10 No generation                                                                         No generation                                                                         7.7   7.4   A little Existence of                   Test                                  delamination                                                                           delamination                   Comparative                                                                          11 No generation                                                                         No generation                                                                         6.9   6.8   A little Existence of                   Test                                  delamination                                                                           delamination                   Comparative                                                                          12 No generation                                                                         No generation                                                                         6.7   7.1   A little Existence of                   Test                                  delamination                                                                           delamination                   Comparative                                                                          13 A little                                                                              Much    59.0  >100  A little A little                       Test      generation                                                                            generation          delamination                                                                           delamination                   Comparative                                                                          14 No generation                                                                         No generation                                                                         5.1   5.2   No delamination                                                                        Existence of                   Test                                           delamination                   Comparative                                                                          15 A little                                                                              Much    14.2  25.0  No delamination                                                                        No delamination                Test      generation                                                                            generation                                                  Comparative                                                                          16 Much    Much    >100  >100  No delamination                                                                        No delamination                Test      generation                                                                            generation                                                  __________________________________________________________________________

In further contrast, the delamination of noble metal layers generatedduring the long time periods of use in every case of Comparative TestNo. 10, in which the Ni plating was carried out after the conventionalelectrolytic activation, of Comparative Test No. 11, in which Au wasplated without the Ni plating after the same activation, of ComparativeTest No. 12, in which Ni content was less than 0.1 g/l in the cathodetreatment, and of comparative Test No. 14, in which the Ni plating layerwas less than 0.5μ in thickness after the cathode treatment. It was seenthat products were inferior in with processability in Comparative TestNo. 13, in which the content of free hydrochloric acid was less than 30g/l in the cathode treatment, as well as in Comparative Test No. 15, inwhich Ni plating layer was more than 0.5μ in thickness after the cathodetreatment.

By the way, in Comparative Test Nos. 10 and 11, which were of classes ofthe conventional processes, the adhesiveness was already insufficienteven immediately after the plating.

<2> Example Nos. 1 and 8 of Example <1> and also comparative Test Nos.11 and 14 for the comparison were repeated, in which a Cu plating of 1μwas carried out instead of the final Au plating using a bath containing:

CuCN

KCN

NaOH

The products were tested in the same way as to the adhesiveness. Resultswere shown in Table 5.

No delamination was generated in Examples of this invention but, incontrast, the delamination was generated in the passage of time in theevery case of Comparative Test Nos. 18 and 19, corresponding to theconventional examples.

                                      TABLE 5                                     __________________________________________________________________________                Pre-Treatments                                                                Before the Metal                                                                       Adhesiveness                                                      No.                                                                              Plating  Before the Test                                                                        After the Test                                  __________________________________________________________________________    Example of the                                                                         16 The same as in                                                                         No delamination                                                                        No delamination                                 Present Invention                                                                         No. 1                                                             Example of the                                                                         17 The same as in                                                                         No delamination                                                                        No delamination                                 Present Invention                                                                         No. 8                                                             Comparative                                                                            18 The same as in                                                                         No delamination                                                                        Existence of                                    Test        No. 11            delamination                                    Comparative                                                                            19 The same as in                                                                         No delamination                                                                        Existence of                                    Test        No. 14            delamination                                    __________________________________________________________________________

<3> SUS 631 (Hv.510) or use as a spring having thickness of 0.08 mm wasused. This material was subjected to the various treatments shown inTable 6 after it was electrolytically degreased with NaOH. Various testswere carried out as to the product and the results shown in Table 7 wereobtained.

In the Tables, the repeated bending was sought by that the testspecimens, in a tape figure of 5.0 mm in width, were put between theholding parts of a tool giving the bending diameter of zero, and fixedand that after a load of 750 gr was attached at the other end, the tapewas repeatedly bent alternatively to the left and right giving eachright angle to count the times to rupture. The processed specimens whichwere the same as in the above <1> were kept at 40° C. for 48 hours in achamber having 200 ppb of NO₂, 100 ppb of H₂ S, 300 ppb of Cl₂ and 75%of hydrogen and maintained at 40° C. to measure the electric contactresistance and the measurement was carried out in the same way. As tothe adhesiveness, the test was the same as in <1>.

                                      TABLE 6                                     __________________________________________________________________________              First Step                        Second Processing                                                   Current   Metal                                       HCl NiCl.sub.2                                                                        Ni  Co          Density                                                                            Time Plating                                                                            Plating                                                                           Thickness                                                                           Metal                     No.                                                                              (g/l)                                                                             (g/l)                                                                             (g/l)                                                                             (g/l)                                                                            Anode    (A/dm.sup.2)                                                                       (second)                                                                           Bath Metal                                                                             (μ)                                                                              Plating            __________________________________________________________________________    Present                                                                              17 150 --  6   -- SUS 631  50   10   A    Ni  0.5   Au                 Invention                                                                     Present                                                                              18  15 --  16  -- Fe--25 Ni                                                                              25   25   "    "   "     "                  Invention                                                                     Present                                                                              19 120 --  5   2.5                                                                              Fe--30 Ni--15 Co                                                                       "    "    "    "   "     "                  Invention                                                                     Comparative                                                                          20 150 --  --  -- Pt       2.0  90   --   --  --    "                  Test                                                                          Comparative                                                                          21 --  --  --  -- Pt       "    "    A    Ni   0.02 "                  Test                                                                          Comparative                                                                          22 120 --  5   2.5                                                                              Fe--30 Ni--15 Co                                                                       30   0.5  "    "   0.5   "                  Test                                                                          Comparative                                                                          23 "   --  "   "  "        150  9.0  "    "   "     "                  Test                                                                          Comparative                                                                          24 120 120 --  -- Ni       1.5  60   --   --  --    "                  Test                                                                          __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________              Repeated Bending                                                                         Electric Conduct                                                   (Times)    Resistance (μ)                                                                         Adhesiveness                                           Before the                                                                          After the                                                                          The Height  Before the                                                                           After the                                    No.                                                                              Test  Test of 0.3 mm                                                                           0.6 mm                                                                              Test   Test                                  __________________________________________________________________________    Present                                                                              17 530   500  8.1   8.6   No     No                                    Invention                        delamination                                                                         delamination                          Present                                                                              18 "     500  8.7   8.0   No     No                                    Invention                        delamination                                                                         delamination                          Present                                                                              19 "     510  8.5   9.6   No     No                                    Invention                        delamination                                                                         delamination                          Comparative                                                                          20 "     390  75    22    A little                                                                             Existence of                          Test                             delamination                                                                         delamination                          Comparative                                                                          21 "     410  70    30    A little                                                                             Existence of                          Test                             delamination                                                                         delamination                          Comparative                                                                          22 "     530  --    --    Delamination                                                                         --                                    Test                                                                          Comparative                                                                          23 "     410  >100  >100  No     No                                    Test                             delamination                                                                         delamination                          Comparative                                                                          24 "     440  >100  >100  No     No                                    Test                             delamination                                                                         delamination                          __________________________________________________________________________

The Results of the adhesiveness are obvious as in the above stated <1>and <2>.

In No. 22, in which processing time of the first step of this inventionwas insufficient, the failure of good adhesion occurred at the finishingof the plating. In Comparative Test Nos. 20 and 21, in which theconventional electrolylic activation was carried out, the repeatingtimes of the bending were greatly reduced, because SUS 631 was a SUS ofthe deposition cured type having a martensite system. This was caused bythe hydrogen embrittlement. In Comparative Test No. 24 of the Ni-strikeplating, this value was considerably reduced. However, this was causedby the hard Ni plating layer (about 1μ) which generated cracks on thesurface rather than by hydrogen embrittlement. In contrast, thisreduction stayed at low levels in the Examples of this invention. Thesame may be obvious from the above as to the electric contactresistance.

In contrast, in No. 23, in which the current density of the first stepof this invention was excessively increased, the decrease in therepeating times of bending was significant and, in addition, the rapidincrease in the electric contact resistance was caused by the cracksformed in the pressure-processing. It is presumed that these are resultsof the absorption of large amounts of hydrogen and the deposition ofhard metal alloy layers.

<4> (Experimental Examples

In order to investigate causes of the difference in the adhesivenessmeasured immediately after the metal plating in the above Example <3>,samples which were obtained immediately after the first steps in Nos.17, 19 and 20 were washed with water and dried. After 4 hours, they weresubjected to the AES(Auge Electron Analysis) to assay the surface depth,from which analysis of oxygen in the depth of repassivated films wereactually measured to give the values of 15, 12 and 65Å, respectively. Itmay be obvious that the repassivation remarkably proceeds in theconventional process, as compared with the process of this invention.

Merits of the Invention

As explained above, metal plated SUS's which are excellent inprocessability and have good adhesiveness, can be produced according tothis invention and, therefore, this invention exerts industriallyremarkable effects, such that the hinderance in quality and performance,which heretofore has caused problems when the materials are used inprecision instruments or electronic instruments.

What is claimed is:
 1. A process for metal plating a stainless steelmaterial comprising (1) a first step of subjecting the stainless steelmaterial to an electrolysis treatment using the stainless steel materialas a cathode in an aqueous solution consisting essentially of free HClin an amount at least 30 g/l and at least one species of Ni and Co ionsin an amount at least 0.1 g/l; (2) a second step of metal plating thesame by Ni or an alloy of Ni in a weakly acidic Ni plating bath; and (3)a third step subsequently plating the same by a noble metal or an alloythereof.
 2. The process as set forth in claim 1, wherein the aqueoussolution contains at least one species of Ni and Co ions in an amountranging from 0.1 to 15 g/l.
 3. The process as set forth in claim 1,wherein a ferroally containing at least one species of Ni and Co in anamount ranging from 1 to 50% is used as an anode in the electrolysistreatment of the first step.
 4. The process as set forth in claim 3,wherein a stainless steel is used as the anode.
 5. The process as setforth in claim 3, wherein the electrolysis treatment of the step 1 iscarried out at a current density on the cathode being in a range of 1 to10 A/cm² and for a time ranging from 1 to 180 seconds; and a platingbath having a pH value ranging from 2 to 4 is used as the bath forplating Ni or an Ni alloy.
 6. The process as set forth in claim 1,wherein the electrolysis treatment of the step 1 is carried out at acurrent density of the cathode being in a range from 1 to 100 A/dm² andfor a time ranging from 1 to 180 seconds.
 7. The process as set forth inclaim 1, wherein a plating bath having a pH value ranging from 2 to 4 isused in the second step.
 8. The process as set forth in claim 7, whereina plating bath containing Ni sulfamate as the main component is used. 9.The process as set forth in claim 1, wherein a plating layer of Ni or anNi alloy is formed, whereby the plating layer is maintained in athickness ranging from 0.05 to 0.5μ in the second step.